Arbuscular mycorrhizal (AM) fungi extensively invade host root tissues. This raises the question of how host plants contend with them; they must exert some kind of control over fungal proliferation since it is confined to a specific root tissue, the parenchymal cortex. Defence processes, which are triggered as a general plant response to microbial invasion, are modulated during root-fungus interactions in arbuscular mycorrhizas. This chapter presents an up-dated review of data on plant defence elicitation in these symbiotic systems and discusses possible mechanisms whereby defence reactions are maintained at a low level, as well as their implication in the phenomenon of bioprotection by AM fungi against soil-borne pathogens.